1. Field of the Invention:
The present invention relates to a developing apparatus which develops an electrostatic latent image formed on the surface of an image retaining member into a visible image by the use of a developer including both a magnetic carrier and a toner.
2. Description of the Prior Art:
A typical conventional image forming system, such as an electrophotographic system or an electrostatic recording system, has at least a charging apparatus, an exposure apparatus and a developing apparatus which are disposed around an image retaining member having a photoconductive material layer or dielectric layer formed on the surface thereof. In the case where the system includes a transfer process, the system is further provided with a transfer apparatus for transferring a developed image to a transfer sheet and a cleaning apparatus for removing the toner remaining on the surface of the image retaining member after transfer has been effected. In addition, the system has a fixing apparatus for obtaining a final image.
In the above-described image forming system, it is common to use a developing apparatus of the type, from the viewpoint of the quality of the developed image, wherein a mixed powder of a magnetic carrier (referred to simply as "carrier", hereinafter) and a toner is employed as a developer, and a magnetic brush is formed on a non-magnetic sleeve having a permanent magnet member therein to brush the surface of the image retaining member, thereby to develop an electrostatic latent image into a visible image. In this type of developing apparatus, the developer which has been used in one developing operation is once scraped off the non-magnetic sleeve and is subjected to detection of the toner density therein before the developer is used again in a subsequent developing operation. The developer is then mixed with an amount of the toner which is replenished in accordance with the detected toner density and is supplied for development again. In such case, when the developer is used for a long period of time, the carrier becomes fatigued such as to deteriorate triboelectric properties. For this reason, the carrier is periodically replaced. In addition, it is necessary to replenish the toner periodically, since it also is consumed.
Thus, in the conventional image forming system, it is necessary to frequently undertake replacement of the carrier and replenishment of the toner. Additionally, it is necessary to replace the image retaining member (photosensitive member), a cleaning member, etc.
Therefore, in order to facilitate maintenance of the image forming system, means have been proposed such as to integrate the replaceable members including expendable supplies or to allow replacement of a recording member (photosensitive member) and replenishment of the toner to be effected simultaneously and at an appropriate timing.
Even when the above-described various improvements have been made, however, it is still necessary to effect replacement of the carrier and replenishment of the toner; therefore, it is not possible to attain a practically maintenance-free developing apparatus.